Communication systems are known to comprise a plurality of base sites that provide communication services to remote units located in corresponding service coverage areas of the base sites. One known communication system is a cellular communication system, such as the Narrowband Advanced Mobile Phone Service (NAMPS). In an NAMPS system, a remote unit (e.g., a mobile or stationary remote unit) that desires to communicate, sends a channel request signal and identification (ID) information to a base site serving the coverage area in which the remote unit resides. Upon receiving the remote unit's ID and channel request signal, the serving base site allocates a communication resource for the remote unit. The communication resource comprises a coordinated pair of frequencies (i.e., an uplink frequency and a downlink. frequency sometimes referred to as voice or traffic channels). In a communication system employing a Time Division Multiple Access (TDMA) protocol, the communication resource comprises a coordinated pair of time slots and frequencies (i.e., a first time slot at an uplink frequency and a second time slot at a downlink frequency). The uplink frequency supports transmissions from the remote unit to the serving base site, whereas the downlink frequency supports transmissions from the serving base site to the remote unit.
Upon allocating the communication resource, the base site sends a channel designation signal containing the uplink and downlink frequency, to the remote unit via a control channel. Upon receiving the channel designation signal, the remote unit tunes its transmitter and receiver to the designated frequencies and begins communicating with a telephone network subscriber or another remote unit via the serving base site. The serving base site then tracks billing information regarding the call, and utilizing the remote unit's ID, charges the appropriate fees to the corresponding caller.
During communication with the base station, a fraudulent remote unit (i.e., a fraudulent user operating a remote unit) can hijack a voice channel by transmitting on the corresponding uplink frequency at a high enough power level, causing the legitimate remote unit to be abandoned in favor of the fraudulent remote unit. Once communication has been established between the fraudulent remote unit and the communication system, the fraudulent remote unit may then utilize three-party calling features of the communication system to place other calls utilizing the legitimate remote unit's ID, causing associated fees to be charged to the legitimate remote unit.
Thus a need exists for a method and apparatus for detection of fraudulent users in a communication system.